1. Field Of The Invention
This invention relates generally to the field of integrated circuit manufacturing and, more particularly, to photolithographic techniques for use in the field of integrated circuit manufacturing.
2. Background Of The Related Art
As most people are aware, an integrated circuit is a highly miniaturized electronic circuit that is typically designed on a semiconductive substrate. The design and fabrication of an integrated circuit usually follows certain steps. First, engineers design an electrical circuit schematic, which defines the circuit's components, as well as their connectivity and general functionality. Engineers typically rely on computer simulation tools to create this circuit schematic and to simulate the electrical function that the circuit is intended to perform.
Once the schematic is complete, the circuit must be translated into a physical representation, commonly called a "layout." Computer aided design tools typically are used by layout designers to translate the schematic circuit into the shapes that will actually embody the devices themselves in the completed integrated circuit. These various shapes make up the individual components of the circuit, such as the transistors, isolation regions, and interconnections. The layout designs are typically quite complicated, especially for integrated circuits having upwards of 100,000 devices on a single semiconductor chip. Indeed, although a finished integrated circuit may appear to be flat, integrated circuits are actually multi-layered devices. Accordingly, the layout for each level must be carefully designed to create such a functional three dimensional circuit.
After the layout of the circuit has been completed, the layout must be transferred onto a semiconductive substrate in order to fabricate the integrated circuit. To accomplish this task, a process called "photolithography" is typically used. In a photolithographic process, the layout design for each level is transferred onto one or more photomasks. A typical photomask is a transparent substrate, such as quartz, which includes thin films of metal or other opaque materials that are formed on the substrate. These opaque materials prevent radiation, such as light or x-rays, from passing through the selected portions of the substrate. The resulting pattern created by the transparent and opaque regions defines the layout.
To transfer the layout design from the photomask onto the semiconductor substrate, a photosensitive polymer film, called photoresist, is normally applied to the semiconductive substrate. The photomask is located between the photoresist and an exposure tool, which is a selected source of light or other types of radiation. The radiation passes through the transparent portions of the photomask and causes the corresponding irradiated portions of the photoresist to change solubility. After exposure, the semiconductive substrate is treated to develop the photomask images that have been transferred onto the photoresist. This treatment typically involves the application of a solvent onto the photoresist layer to remove the soluble portions of the photoresist. The photoresist mask that is created as a result of this developing procedure is then used to create certain device features of the circuit on the semiconductive substrate.
Photolithography has been used for many years in the fabrication of integrated circuits. As integrated circuits have become smaller and more densely packed, the photolithographic processes used to fabricate such circuits have undergone a number of changes and improvements to meet the ever increasing demands for fabricating such circuits. For instance, resolution is one of the limiting characteristics of the exposure tool. The resolution of an exposure tool is typically defined as the minimum feature that the exposure tool can repeatedly expose onto the photoresist. Currently, there is a need to produce elements of electrical circuits at sizes that are close to or below the resolution limit of available exposure tools. Indeed, various methods have been developed for creating certain features that are actually smaller than currently achievable photolithographic resolution. However, even excluding situations where such extremely small features are desirable, as other critical dimensions of the layout become smaller and approach the resolution limit of the photolithographic process, it becomes increasingly difficult to reproduce the photomask layout onto the photoresist in a consistent manner.
It has been found that differences in the development of circuit features on the photoresist depends on the proximity of the features to one another. In photolithography, this occurrence is typically called the "proximity effect." The proximity effect causes a feature on a photolithographic mask to be patterned into a photoresist differently depending on whether the feature is isolated from or adjacent to other features on the mask. The proximity effect results from diffraction in the photolithographic exposure tool, and it may also result from photoresist processing or subsequent etching. Specifically, the radiation diffracts around adjacent opaque features on the mask and interacts in such a way as to produce pattern dependent variations. In other words, when two features are in close proximity to one another, the diffracted radiation integrates and combines.
For example, lines designed to have the same dimension will transfer onto the photoresist differently depending upon whether the lines are isolated from one another or densely packed together. If these lines are relatively isolated from one another, so that the proximity effect is not a concern, the lines tend to transfer generally as designed. However, if the lines are densely packed together, the proximity effect causes the photoresist to receive some radiation between each line due to the diffractive interactions. Accordingly, because the proximity effect causes more of the photoresist to be exposed to the radiation, the group of densely packed lines tends to transfer differently onto the photoresist when compared with the isolated line.
Numerous methods have been developed in attempts to overcome the proximity effect. One method for reducing the problems caused by the proximity effect involves the biasing of the photomask. The dimensions of the layout design on the photomask are selectively adjusted to precompensate certain mask features so that the final target dimensions that are transferred onto the photoresist are consistent with the non-compensated features. Continuing the example mentioned above, it has been found that, under certain exposure conditions, the final line width for isolated features may be larger than the final line width of densely packed features when utilizing a positive photoresist. To reduce the inconsistency, designers have created mask patterns in which the original mask line width of the isolated features is smaller than the original mask line width of the densely packed features. By precompensating the mask pattern in this fashion, the final line width created on the photoresist for the isolated features and for the densely packed features is approximately the same.
Unfortunately, this biasing technique exhibits certain disadvantages. One problem with the biasing approach is that it requires empirical trial and error and is heavily dependent on the exposure tool, the photoresist, and the developer. Consequently, variations in any of these factors can have an adverse impact on the biased features. Such an impact typically results in the need for mask revisions, which are very expensive and have long lead times that can drastically affect production schedules. Another problem of the biasing approach is the fact that increasing the thickness of the photoresist reduces the impact of the proximity effect caused by the photoresist process. As a result, at a certain photoresist thickness, the proximity effect can be minimized for a given exposure tool. However, by increasing the thickness of the photoresist to compensate for the proximity effect, the photoresist becomes too thick to create features at the resolution limit of the photolithographic process. Thus, biasing typically offers an unsatisfactory solution to problems caused by the proximity effect.
The present invention may be applicable to one or more of the problems discussed above.